There is a good evidence that myosin has associated with it noncovalently, small molecular weight proteins, light chains (MLC) of about 20,000 daltons, and that the association is necessary for the biological activity (ATPase and actin binding) of the contractile protein. Research by this investigator has shown that myosin main, heavy chains (MHC) are synthesized separately from and at different rates than MLC, implying they are turning over at different rates. The proposed research will utilize developing muscle to examine in more detail the distribution of MLC in muscle and in addition several aspects of the relative metabolism of the two myosin components. Studies will be of chick embryonic muscle development in ovo and also of cultured chick embryonic cardiac and skeletal muscle. The experiments will be concerned with: 1/ whether the relative rates of MHC and MLC synthesis vary during embryonic development of muscle and in cultured muscle; 2/ whether there are heterogeneous rates of synthesis of the MLC of a given muscle; 3/ whether MLC are found in significant amounts in muscle sarcoplasm unassociated with MHC and if so what relationship their metabolism bears to that of MHC-associated MLC. Metabolic studies will involve analyses of the uptake of radioactive amino acids into protein. In addition, antibodies will be made against each of the MLC associated with myosin and used in radioimmunoassay procedures to determine their distribution in muscle and in the metabolic studies of the specific activity of previously labeled MLC. Myosin ATPase is probably the rate-limiting step in the contractile process. Hence, an understanding of the relationship between the distribution and metabolism of its components is of crucial importance to understanding the assembly of myosin in developing and adult tissues, and to an understanding of the maintenance of normal muscle contractility.